This invention relates to a belt drive device and an image forming apparatus having the belt drive device.
A conventional electrophotographic image forming apparatus uses an endless belt for feeding a recording media to an image forming portion in which an image is formed on the recording media. Such an image forming apparatus includes a belt drive device having the endless belt adjacent to the image forming portion. The belt drive device feeds the recording media to the image forming portion by moving the endless belt that absorbs the recording media, so that the image is formed on the recording media in the image forming portion.
The belt drive device includes two rollers (provided in parallel to each other) around which the endless belt is stretched, and drives at least one of the two rollers to thereby move the endless belt. However, there is a possibility that a skewing of the endless belt may occur, depending on a parallelism between the two rollers, an imbalance of tension applied to the endless belt, a dimensional accuracy of the rollers, or the like.
In order to prevent the skewing of the endless belt, there is proposed a belt drive device having a pulley coaxially provided on the roller. The pulley is rotatable about the rotation axis of the roller, but does not move in the axial direction of the roller. Further, a bead is provided on the inner circumference of the lateral end of the endless belt, and engages a groove formed on the pulley. Such a belt drive device is disclosed in, for example, Japanese Laid-Open patent publication No. 2002-23518.
However, since the conventional belt drive device prevents the skewing of the endless belt by means of the engagement between the bead and the groove of the pulley, the friction between the bead and the pulley may increase when the force of the skewing increases. In such a case, the bead may be worn, and the worn bead may move out of the groove of the pulley, with the result that the endless belt may be dropped out of the pulley. Once the endless belt is dropped out of the pulley, the belt drive device does not recover its original condition by itself, and therefore an expensive replacement of belt drive device is inevitable.